With a conventional or current optical transmission system, transmission speed (bit rate of data) and total transmission capacity (transmission speed per one channel×the number of channels) and the maximum distance the transmission can reach are dependent on the optical signal-to-noise ratio (optical S/N), waveform distortion, and phase distortion of the optical signal.
The waveform distortion and the phase distortion of an optical signal are highly dependent on chromatic dispersion (including high-order dispersion) and nonlinear optical effects and the like of transmission-path optical fibers. The optical S/N is dependent on amplified spontaneous emission noise (hereinafter, “ASE noise”), caused by an optical amplifiers for substituting loss in optical fibers, and noise characteristics and the like within a transmitting and receiving unit.
As for the transmission waveform distortion due to the chromatic dispersion, there has been developed both a transmission path in which normal dispersion fibers and anomalous dispersion fibers are provided alternately, and a compensation technology that uses a wavelength-dispersion compensating unit such as dispersion-compensating fibers or electric signal processing on the reception side, and the like.
Furthermore, there has been developed a practical system where the attenuation of power of an optical signal due to the loss in transmission-path fibers is compensated by optical amplifiers such as optical fibers and amplifiers, the system thereby allowing signals to be transmitted across oceans as a single-wavelength transmission or multiple-wavelength transmission at 10 Gb/s.
Japanese Laid-open Patent Publication No. 2006-184851 discloses a technology with which a control pulse of a different wavelength from that of the optical signal is generated, the generated control pulse and the optical signal are input to nonlinear optical fibers, and then the optical signal is parametrically amplified along the polarization direction of the control pulse, so that high-speed switching with high-switching efficiency over a sufficiently wide range of wavelengths can be realized.
The current problem to be solved is how to realize long-distance transmission with optical signals at high speed, e.g., at 40 Gb/s or 160 Gb/s. However, even with highly accurate dispersion compensation and qualified optical amplifiers being used conventionally, waveform distortion that remains and the decrease of optical S/N ratio caused by the ASE noise due to the addition by the optical amplifier are significant so that the practical distance of transmission is limited.
The significant problem is to shape the distorted waveform, correct the phase distortion, and suppress the accumulated ASE noise, so as to realize long-distance optical-fiber transmission with high-speed optical signals (including optical signals with wavelength-division multiplexing).